Virtual inheritance places the base subobject at a dynamic offset, but
load_impl Case 2a uses reinterpret_cast which assumes a fixed offset.
This caused segfaults when dispatching inherited methods through virtual
bases (e.g. SftVirtDerived2::name()).
Add an is_static_downcastable SFINAE trait that detects whether
static_cast<Derived*>(Base*) is valid. When it is not (virtual
inheritance), set multiple_inheritance = true in add_base to force the
implicit_casts path, which correctly adjusts pointers at runtime.
Remove the workaround .def("name", &SftVirtDerived2::name) from
test_smart_ptr.cpp that was papering over the issue.
Made-with: Cursor
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